Mitochondria are dynamic membrane organelles that undergo division and fusion. The balance between these opposing activities plays a critical role in controlling mitochondrial structure and function. Mitochondrial division and fusion are mediated by conserved dynamin-related GTPases including Drp1 for division and Opa1 for fusion. Inhibition of mitochondrial division enlarges mitochondria due to ongoing fusion while inhibition of fusion fragments mitochondria. Abnormalities in mitochondrial division and fusion are associated with many neurodegenerative diseases such as autosomal dominant optic atrophy, Charcot-Marie- Tooth neuropathy, Alzheimer's disease, Huntington's disease, and Parkinson's disease. Understanding the pathogenesis of these diseases requires a deeper knowledge of the physiological functions of mitochondrial dynamics. In this proposed research, we will determine how altered mitochondrial dynamics causes neurodegeneration. Specifically, we will test two untested hypotheses. The first hypothesis states that mitochondrial dynamics directly regulates mitochondrial functions. In this model, mitochondria must continuously divide and fuse to maintain their functions for survival of neurons. The second hypothesis states that normal mitochondrial structures, but not dynamics per se, are critical in neurons. In this second model, mitochondrial division and fusion are dispensable if mitochondria can maintain their structure independent of mitochondrial dynamics. To attack this problem, we have developed a straightforward approach by introducing mitochondrial stasis in postmitotic neurons in mice. In these animal animals, mitochondrial division and fusion can be individually or simultaneously inhibited using Cre/loxP-mediated knockout for Drp1 and Opa1. We have shown that neurons lacking both Drp1 and Opa1 restore normal mitochondrial morphology, allowing us to block mitochondrial dynamics without affecting mitochondrial structures and clearly distinguish these two hypotheses described above. Therefore, this study will provide novel insight into the pathogenesis of many familial and sporadic neurological disorders.